The chemical and petrochemical industries, in particular, are major consumers of energy in the generation and processing of their products. The optimization of the existing processes requires an improved ability to monitor, among other parameters, the chemical composition of the reactor streams. The sensors needed to provide this information must be sufficiently robust so that they can withstand harsh chemicals at high temperatures and, in some situations, function in the presence of solids. Optical techniques, especially IR and Raman spectroscopies, have always played an important role in process control in the chemical industry. IR and Raman measurements provide complementary information on the chemical composition of a reactor stream, in so far as the terminal vibrational levels of the molecules accessed by the two types of transitions do not overlap in general. In other words, bands that are strong in IR absorption are typically weak in Raman scattering, and vice versa. Furthermore, the instrumentations for these two kinds of spectroscopy are vastly different and separate probes must be used for their respective implementations.
Fiber-optic absorption sensors in the near-IR first made their way into chemical plants in the 1990's. In a typical system light is guided by an optical fiber to a probe, where it is collimated and sent out a window. After traversing a gap through which the reactor stream flows, it is passed through a second window and refocused into a second fiber which carries it to the detector. These probes are now widely used in the observation of overtone and combination bands as part of process control in the chemical industry.
An evanescent wave mid-IR absorption sensor between 2.5 μm and 11.5 μm has been demonstrated using a chalcogenide glass fiber (see J. S. Sanghera, F. H. Kung, P. C. Pureza, V. Q. Nguyen, R. E. Miklos, and I. D. Aggarwal, “Infrared evanescent-absorption spectroscopy with chalcogenide glass fibers,” Appl. Opt. 33, 6315-(1994)).
Fiber-optic Raman probes generally use one delivery fiber to guide the excitation light to the sample and one or more collection fibers to carry the Raman scattered light to the spectral analyzer. Note that this geometry is in contrast to the one described earlier for the conventional IR absorption probe, wherein electromagnetic energy passes through the sample reactor as well as the evanescent wave absorption probe.
What is needed, however, is a fiber-optic Raman probe capable of the simultaneous determination of IR and Raman spectral features to provide complimentary information on the chemical system being analyzed.